In recent years, consumer health concerns have increased market demand for safe and nutritious foods. Leafy vegetables are a rich source of calcium, iron, β-carotene, AsA (vitamin C), and dietary fiber. They contain more micronutrients per mouthful than any other food (Gupta and Prakash, 2008). Plants can synthesize some antioxidants including, for example, AsA, glutathione, α-tocopherol (vitamin E), and polyphenol to protect themselves against oxidative damage caused by environmental stressors (Blokhina et al., 2003; Romani et al., 2002). These compounds evolved to detoxify reactive oxygen species in plants, but they also show beneficial activity against some human diseases related to oxidative damage and aging (Fusco et al., 2007; Iwai, 2008; Nicolle et al., 2004). Multiple studies have reported increasing antioxidant levels in plants via intentional environmental stress, such as drought, temperature, and high light intensity (Hamilton and Fonseca, 2010; Johkan et al., 2010; Oh et al., 2009; Sharma and Dubey, 2005). However, these techniques to improve plant quality are hard to implement in agricultural fields because of the risk of overstress, which has adverse effects on plant growth and yield (Hidaka et al., 2008). A plant factory is an efficient and stable system in which plants are cultivated in a controlled environment year-round. The products are chemical-free, fresh, and clean (Takatsuji, 2010). For the success of industries based on hydroponically grown plants, it is not only important to increase yields but also to improve quality. Improvements to quality attributes such as flavor and/or nutritional value can differentiate hydroponically grown products from field-grown products. The environmental control techniques in a plant factory make it possible to alter these characteristics by treating plants with moderate and consistent levels of stress.
Hydroponic culture is currently the main cultivation method in plant factories. In hydroponic culture, some types of stress can be easily introduced to the roots. Stress to the roots causes declines in root absorption and leaf photosynthesis and results in both osmotic and oxidative stress to the whole plant (Kitano et al., 2008). For example, irrigating tomato (Solanum lycopersicum) plants with salt water is commonly practiced and produces high-quality fruits enriched in sugar and minerals (Cuartero and Fernández-Muñoz, 1999). Spinach (Spinacia oleracea) plants that were stressed with cold treatment of their roots had increased levels of beneficial sugar, AsA, and ferric ions (Fe2+) in their leaves as well as decreased levels of nitrate ions (NO3−) and oxalic acid, which are harmful substances (Hidaka et al., 2008). However, in these reports the growth of the aerial parts of these stressed plants declined, highlighting the challenges that overstressing plants pose to commercial cultivation. An alternative cultivation method is necessary to increase quality without reducing yield.
Ideally, an environmental stress treatment for commercial production would have a low cost. Drought stress by suddenly exposing part of the roots to air can be practiced simply by lowering water levels in hydroponic culture and does not require extra equipment, such as ultraviolet lamps, or alteration of other conditions in the culture room, equipment, or solutions. In addition, this method might allow easier fine-tuning of stress levels than alternative methods. New cultivation systems that use less water than conventional hydroponics systems have been developed as an economical, low-maintenance method for growing lettuce (Kratky, 1993, 2004). In this system, roots are exposed to airspace during the entire growth period and absorb nutrients from a noncirculating nutrient solution in a tub. However, to date, there have been no studies on optimization of conditions or short-term treatments to increase the nutritional quality of plants grown in this system. In this study, we demonstrate that a limited drought stress treatment to part of the root mass enhances the nutritional attributes of leafy vegetables without damaging growth.
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